Ring Enlargement of polyhydroxylated pyrrolidines to piperidines by Mitsunobu reaction: A fortuitous synthesis of 1-Deoxy-L-Allonojirimicina

Chiral 3,4-dibenzyloxy-5-hydroxymethyl-2-thiazolylpyrrolidines under Mitsunobu conditions afforded the corresponding R-protected pyrrolidines and 2-deoxy-piperidines in different ratios depending on the stereochemistry of the starting pyrrolidine and the nature of the acid R-OH. A mechanistic scheme is proposed involving the formation of an aziridinium ion as an intermediate.